This invention relates to horizon sensors using optical radiation for attitude control of bodies moving outside of the earth's atmosphere, and more particularly, to such sensors which are protected from being damaged by laser radiation.
Horizon sensors are frequently used for attitude control of spacecraft. The horizon represents a line of discontinuity between the earth's atmosphere and outer space which discontinuity is very marked particularly from high altitude and is characterized by a large difference in infrared radiation. Outer space is extremely cold and its infrared radiation is very low while the earth and the earth's atmosphere are relatively much warmer. The horizon sensor scans this line of thermal discontinuity with the abrupt change of optical radiation on either side of it and generates an electrical signal by continuously scanning a detector across the horizon to produce an electrical output in the form of square waves or regularly repeating rectangular pulses which are used with reference pulses as the scan passes predetermined points in the moving body in which the horizon sensor is positioned. By comparing the intervals between horizon crossings and the reference pulses an output signal is produced which gives information of the attitude of the vehicle with respect to the horizon.
Conical scan horizon sensors cause the small field of view of an infrared detector which may be around 1.degree. or 2.degree. in diameter to scan a large cone in space with a half angle of 30.degree. or more which crosses the earth's horizon at two points. From the rectangular wave shape produced at the output of the infrared detector, the direction toward the center of the earth can be determined. An example of such a horizon sensor is shown and described in U.S. Pat. No. 3,020,407.
The aforesaid patent is a conical scan horizon sensor having a rotating optical assembly which scans a cone across the earth's horizon and applies the radiation received therefrom from a field of view to the infrared detector. The optical assembly and detector ca also be stationary with the scanning accomplished by the rotation of the satellite or another rotating element on the satellite to which the sensor is secured. The detectors which are used in horizon sensors are small, delicate and normally measure small amounts of radiation. In addition many horizon sensors use interference filters comprised of multiple thin film layers for only supplying predetermined radiation to the infrared detector, for example, radiation in the carbon dioxide band which band is commonly used in horizon sensing because of its uniform distribution around the earth. Thin film interference type filters have an extremely low heat capacity and can be easily damaged by high energy levels. There is currently increasing concern about horizon sensors being damaged by laser radiation because of the high energy levels contained in the laser radiation and due to the delicate components such as the detectors and/or interference filters which are, used in the horizon sensors. The threat of laser radiation occurs at wave lengths shorter than 14 microns at which the horizon sensors usually operate. Accordingly, it is essential to attenuate laser radiation impinging on the interference filters, detectors or other more fragile components of the horizon sensor.